A magnetostrictive linear position detector typically includes a magnetostrictive waveguide wire which is housed in a protective waveguide housing about which a magnet is slidingly engaged. A current pulse is sent through a wire near the waveguide (or through the waveguide itself), and this pulse interacts with the magnetic energy of the magnet to induce a torsional strain wave in the magnetostrictive waveguide at the location of the magnet. The strain wave travels along the length of the waveguide and passes through a mode convertor, such as a pickup coil, which converts the mechanical wave into an electrical signal. To obtain the location of the magnet, the time between the transmission of the current pulse and the reception of the signal from the coil can be measured and converted to a distance, because the speed that the torsional wave will travel along the waveguide is known. Accordingly, when the magnet is connected to a movable mass, such as a liquid level quantity in a storage tank, or a movable element in a machine tool for example, the exact position of the mass can be measured and monitored.
In more advanced magnetostrictive linear position detectors, the ability to set reference points along the measurement stroke is provided. For example, in some such sensors, the magnet can be positioned at any location along the waveguide housing, and a button or buttons can be pressed to save the current position of the magnet in memory so that this position can be used as a reference point. In some systems, the output signal to be provided at this reference point car be assigned, such as by manipulating the programming buttons. Additional reference points can also be assigned and utilized in a similar manner.
Such calibration systems allow the output of the transducer to be changed from the original setup provided by the manufacturer. For example, while the manufacturer might configure the transducer to provide a 0 volt output when the magnet is at one end of the waveguide housing and a 10 volt output when at the opposite end, the ultimate user of the transducer may desire different settings. With such a calibration system, the user could assign any possible voltage output to any of the possible magnet positions. For example, the user may wish that a position 2 inches (50.8 mm) from the first end provides a 0 volt output, and that a position 3 inches (76.2 mm) from the opposite end provides a 10 volt output. By using such a calibration system, these reference points can be assigned the desired outputs. Once the reference points are assigned, the system can then be set up to scale all subsequent magnet positions based upon the reference points.
Accordingly, the programmability or adjustability of reference points allow the user to customize the sensor to provide the desired output range over the desired measurement stroke. Thus, a reference point can be, for example, an endpoint of the stroke.
However, such calibration systems are not without disadvantages. For example, such systems can expose the electronics to potential damage. More specifically, to access the programming buttons, screws or other covers on the electronics housing must be removed, and the buttons can then be depressed by extending a pin or screw driver through the resulting access openings. However, providing such access openings, even when sealed off by screws and the like, can compromise the ability of the housing to seal off moisture and other contaminates which can damage the delicate electronic components inside. In many applications, an excellent watertight seal is required, such an IP67 rated seal, and access openings generally diminish the ability of the housing to achieve and maintain such a seal.
Moreover, if the screw or cover is lost or is not properly replaced over the access opening after the desired programming has been conducted, the seal is again compromised or lost. In addition, the device used to depress the programming buttons, such as a screwdriver, pin, or finger, can carry electrostatic charge which can itself damage electronic components within the housing.
Accordingly, it is desirable to provide a system and method for calibrating a linear position detector which does not affect the ability of the transducer's electronics housing to protect against undesirable ambient and external moisture, contaminates, and electrostatic discharge, and which does not require opening and closing or other physical access through the housing for programming the detector.